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Nature and sources of data

Provisional estimates are based on all complete birth records received and processed by the National Center for Health Statistics (NCHS) as of a specified cutoff date. National provisional estimates include events occurring to U.S. residents within the 50 states and District of Columbia. NCHS receives the birth records and monthly provisional occurrence counts from state vital registration systems through the Vital Statistics Cooperative Program. Provisional quarterly estimates are compared with final data when available (3).

Individual records are weighted, when necessary, to independent provisional counts of births occurring in each state by month. These monthly state-specific provisional counts serve as control totals and are the basis for the record weights used for computing provisional estimates. If the number of complete records is greater than the provisional count received from the state, the state-specific number of complete records is used instead, and the weight is set at 1.

Table I shows the percent completeness of the provisional data by month for the United States and each jurisdiction based on where the births occurred. The percent completeness is obtained by dividing the number of complete records from each state for each month by the corresponding provisional count and multiplying by 100. The record weights described above are calculated by taking the inverse of the percent completeness (divided by 100) by state and month. Although data by place of occurrence are used to compute the weights, all rate estimates are for the residents of the 50 states and District of Columbia.

Table I. Natality data completeness as a percentage of monthly provisional count: United States, each state, District of Columbia, and New York City, Quarter 2, 2018

Area

2018-Jan

2018-Feb

2018-Mar

2018-Apr

2018-May

2018-Jun

Total U.S.

100

100

100

100

100

100

Alabama

100

100

100

100

100

100

Alaska

100

100

100

100

100

100

Arizona

100

100

100

100

100

100

Arkansas

100

100

100

99.9

100

100

California

100

100

100

100

100

100

Colorado

100

100

100

100

100

100

Connecticut

100

100

100

100

100

100

Delaware

100

100

100

100

100

100

District of Columbia

100

100

100

100

100

100

Florida

100

100

100

100

100

100

Georgia

100

100

100

100

100

100

Hawaii

100

100

100

100

100

100

Idaho

100

100

100

100

100

100

Illinois

100

100

100

100

100

100

Indiana

100

100

100

100

100

100

Iowa

100

100

100

100

100

100

Kansas

100

100

100

100

100

100

Kentucky

100

100

100

100

100

100

Louisiana

100

100

100

100

100

100

Maine

100

100

100

100

100

100

Maryland

100

100

100

100

100

100

Massachusetts

100

100

100

100

100

100

Michigan

100

100

100

100

100

100

Minnesota

100

100

100

100

100

100

Mississippi

100

100

100

100

100

100

Missouri

100

100

100

100

100

100

Montana

100

100

100

100

100

100

Nebraska

100

100

100

100

100

100

Nevada

100

100

100

100

100

100

New Hampshire

100

100

100

100

100

100

New Jersey

100

100

100

100

100

100

New Mexico

100

100

100

100

100

100

New York City

100

100

100

100

100

100

New York1

100

100

100

100

100

100

North Carolina

100

100

100

100

100

100

North Dakota

100

100

100

100

100

100

Ohio

100

100

100

100

100

100

Oklahoma

100

100

100

100

100

100

Oregon

100

100

100

100

100

100

Pennsylvania

100

100

100

100

100

100

Rhode Island

100

100

100

100

100

100

South Carolina

100

100

100

100

100

100

South Dakota

100

100

100

100

100

100

Tennessee

100

100

100

100

100

100

Texas

100

100

100

100

100

100

Utah

100

100

100

100

100

100

Vermont

100

100

100

100

100

100

Virginia

100

100

100

100

100

100

Washington

100

100

100

100

100

100

West Virginia

100

100

100

100

100

100

Wisconsin

100

100

100

100

100

100

Wyoming

100

100

100

100

100

100

1 Excludes New York City.

NOTE: The percent completeness is obtained by dividing the number of complete records from each state for each month by the corresponding provisional count and multiplying by 100. NCHS receives the birth records and monthly provisional occurrence counts from state vital registration systems through the Vital Statistics Cooperative Program.

Population denominators

Population estimates used for computing rates are postcensal estimates, which originate from the U.S. Census Bureau and are based on the 2010 census (5). All population estimates used for a single data year are based on population estimates of the same vintage (i.e., the same reference year). Changes in rates between years in part reflect differences between vintages of the population estimates. Quarterly rates are based on the population estimates for the month in the midpoint of the quarter. Rates for 12-month periods are based on the midyear population estimate.

Computing rates

The “12 months ending with quarter” birth rates are on an annual basis per 1,000 estimated population within specified age groups. The general fertility rate refers to the total number of births per 1,000 women aged 15-44. Birth rates for women aged 50 and over have historically been too small to compute reliable age-specific birth rates. Consistent with other reports, birth rates to women aged 45 and over are computed by including all births to women aged 45 and over in the numerator, while the denominator includes population estimates of women aged 44-49 years (1-3). For comparability to the annual birth rate estimates, quarterly birth rates (i.e., a 3-month period) have been annualized to represent births per year per 1,000 population that would be expected if the quarter-specific rate prevailed for the year.

Other rates (e.g., preterm birth rates, term birth rates, cesarean delivery rates) are presented as per 100 births. It is not necessary to annualize these quarterly rates as described above because they reflect percentages of births occurring in a given quarter. Estimates of birth rates at 42 weeks gestation or later (post-term) are not presented due to the small number of births in this group.

Accuracy of estimates

Provisional estimates are subject to some nonrandom sampling error. The quarterly provisional estimates are based on data that is potentially more incomplete for the most recent months. Data are weighted to independent control counts of births, and no imputations have been performed, because it is assumed that the data are missing at random (i.e., the degree of missing data is unrelated to estimates of reproductive health). Estimates of completion rates by month were all above 95% for the United States. However, certain states may have more delayed reporting, and it is unknown whether indicators of reproductive health may be different for these states compared with states having complete reporting. Even if no differential delay occurred, some sampling error would still exist for rate estimates, because they are based on incomplete data. A guideline for the size of this sampling error is given by deriving the variation that would occur if the data were missing at random (6); standard errors for birth rates have been calculated according to these methods, accounting for sampling error.

Partly because of the factors discussed above, provisional estimates of birth rates are rarely higher than the true rate. Historically, provisional estimates of birth rates track closely with estimates based on final data (7). Based on simulations of various levels of data completeness, ranging from 50% through 90%, both quarterly and 12 month-ending estimates of the indicators included in this release can be expected to be within 1-2% of the estimates based on complete data. Exceptions were noted for birth rates among women aged 10-14 and 45 and over under scenarios where data completeness was 80% or lower. For these two age groups, the percent differences between estimates based on complete data and those based on various levels of incomplete data were larger than 1% because the birth rates are small (0.2 and 0.9 per 1,000, respectively). However, the absolute differences between the rates across various levels of data completeness was approximately 0.1 for both of these age groups. For example, estimated birth rates for females aged 10–14 under various levels of completeness ranged from 0.2 through 0.3 per 1,000, while those for women aged 45 and over ranged from 0.8 through 0.9 per 1,000. Notably, these simulations assumed data were missing at random.

Because the timeliness of birth reporting has been improving, accuracy of the estimates may change over time. Estimates for previously released quarters are revised based on new data and updates received since the previous release. As a result, the reliability of estimates for a specific quarter will improve with each quarterly release, and estimates for previous quarters may change with the addition of updated data. Estimates may differ from previously published preliminary data due to rounding, application of different record weights, or the use of updated population estimates.

Interpretation of changes over time

Unless otherwise specified, a difference in age-specific birth rates is reported only if statistically significant at the 0.05 level by the test given in reference 6. That test uses a variance estimate that includes both the random fluctuation in the true number of births and the random sampling error. Similarly, differences in percentages (e.g., cesarean delivery and preterm birth) between the most recent quarter and the same quarter of the preceding year are reported only if statistically significant at the 0.05 level according to methods outlined in reference 8.